1. Field of the Invention
This invention relates to hard drives and to a disk sequencer selecting control word for identifying the position of a data head relative to a track containing headerless data sectors.
2. Description of Related Art
A conventional magnetic media for a hard drive is a disk having major surfaces divided into concentric tracks for data storage. Servo wedges subdivide the tracks into data frames where the portion of a servo wedge within a track is referred to as a servo sector and separates one data frame in the track from another. A servo sequencer uses the servo sectors in a track to keep a data head following the track. Servo wedges are typically spaced about the disk in a spoke-like pattern, and the time interval between the data head passing consecutive servo wedges depends on the angular velocity of the disk and is independent of the track being followed. Thus, a system clock signal that has the same frequency for all tracks controls timing when handling servo sectors or generating end-of-servo (EOS) pulses marking the ends of servo sectors and the starts of data frames.
For constant density recording in the data frames, the disk area per bit of data is constant, but the rate at which a data head encounters the data varies according to velocity of the track being followed. Accordingly, the data transfer rate depends on the radius of the track, and a byte clock signal having a frequency that depends on the track being followed controls timing of data read or written. The data within the data frames are typically organized into data sectors, each of which contains the same amount of data, for example, 512 bytes. The data sectors may include a single data field that is entirely within a data frame or multiple data fields that are separated from each other, for example, by a servo sector. When reading or writing a data sector, the boundaries of each data field (i.e., the start and splits in each data sector) need to be found. One method for finding data fields uses information from a header of a data sector to locate splits in the data sector. However, split information stored in headers wastes disk space that could otherwise store data.
A headerless data sector format lacks headers containing split information which leaves more disk area for data storage. However, a disk sequencer requires split information from a source other than a header. Such split information can be stored in non-volatile memory or on disk in data sectors and then transferred to a data buffer memory. The split information can be stored more compactly in data sectors than in headers because the split information for one data frame often applies to multiple data frames on multiple tracks. Storing the split information in a data buffer typically requires additional data buffer capacity for the split information, bandwidth for flow of split information, and processing power to select and interpret the split information.
Typically, a microprocessor in the disk drive executes firmware that selects the split information that corresponds to the current position of the data head relative to a track being followed. The calculations required to identify the split information can be complex and consume processing power that could otherwise be used for other purposes such as data buffer management, controlling a servo sequencer, handling a host interface, and converting read/write requests to lists of physical data sector to be accessed. If the burden on the microprocessor for alignment of split information is reduced, a less powerful and less expensive microprocessor might be used. Accordingly, processes and/or circuits that simplify the task of selecting the appropriate split information for a disk sequencer are sought.